Beveling allows a full-penetration weld (if required). It gives the torch tip access to deep-lying base metal, prepares a well-defined, uniform container for molten metal and ensures quality welds. Bevel angles and root openings are essential for adequate weld penetration in thick plates or pipes. The joint design must specify all relevant dimensions and angles. the joint design and root opening specification are both part of the weld procedure.
Rivets can be easily removed by “washing” one head off with an acetylene torch. Next, a hole is burned along the rivet longitudinal axis. Then, while still hot, the rivet can be driven out with a pin punch and a two-pound hammer. This process completely removes the rivet and leaves a clean hole in original condition.
Cutting is initiated by heating the edge of the steel to near melting point using the pre-heat jets only, then using the separate cutting oxygen valve to release the oxygen from the central jet. The steel is instantly oxidized into molten iron oxide, producing the cut. Robotic oxy-fuel cutters sometimes use a high-speed divergent nozzle. This uses an oxygen jet that opens slightly along its passage. This allows the compressed oxygen to expand as it leaves, forming a high-velocity jet that spreads less than a parallel-bore nozzle, allowing a cleaner cut. These are not used for cutting by hand since they need very accurate positioning above the work. Their ability to produce almost any shape from large steel plates gives them a secure future in shipbuilding and in many other industries.
The Ideal Cut
The ideal cut is a narrow gap with a sharp edge on either side of the work piece. Overheating the work piece and thus melting through it, causes a rounded edge. Robotic oxy-fuel cutters sometimes use a high speed divergent nozzle. This uses an oxygen jet that opens slightly along its passage. This allows the compressed oxygen to expand as it leaves, forming a high-velocity jet that spreads less than a parallel-bore nozzle, allowing a cleaner cut. These are not used for cutting by hand since they need very accurate positioning above the work.
When oxy-fuel cutting, the metal is heated until it glows orange~ (1800°F= 982°C), and then a lever on the torch is pressed to pass a stream of oxygen through the work-piece to burn the steel away where the cut is desired. The iron-oxide product of this combustion process falls to the floor as dust. Once the process is started properly, there should be no globs of melted steel under the work-piece. No melting should occur.
It is often necessary to begin a cut at some point other than on the edge of a piece of metal. This technique is known as piercing. Piercing requires a larger preheat flame than the one used for an edge start. The tip should be angled and lifted up as the cutting oxygen valve is opened. The torch is held stationary until the cutting jet pierces through the plate.
Oxy-fuel Gas Cutting Machines
Oxy-fuel gas cutting machines are either portable or stationary. Portable cutting machines are primarily used for straight line cutting. They can carry one or more heavy-duty machine torches.
Starting a cut on an edge
To start a cut on an edge, place the preheat flames halfway over the edge, holding the end of the flame cones 1/8” above the surface of the material to be cut. When the top corner reaches a reddish yellow, the cutting oxygen valve is opened and the cutting process begins.
The oxygen fuel gas-cutting torch can be used for gouging. Special gouging tips are available and they should be selected based on the particular geometry of joint preparation. It is possible, by closely watching the cut surface, to find and follow cracks during the flame gouging operation. The edges of the cracks will show since they become slightly hotter.
For a plate thickness of ½” or more, the cutting tip should be held perpendicular to the plate. When cutting a thinner plate, the tip can be tilted in the direction of the cut. Tilting increases the cutting speed and helps prevent slag from freezing across the kerf.